All cells have the ability to detect and respond appropriately to small changes in the concentration of a variety of chemical signals. In the microbial world, bacterial behavior, sporulation, pathogenesis, and gene regulation all incorporate a series of steps that involve signal transduction. We have shown that in bacterial chemotaxis a complex information processing system includes a transmembrane receptor protein which is capable of binding ligand, transmitting information across the membrane and initiating an citation and an adaptation process. Excitation involves the activation or inactivation of a protein kinase and the regulation of phosphorylated levels of the CheY protein. CheY regulates the "effector" element in this system, a "switch" which in turn controls the direction of flagellar rotation. The adaptation mechanism involves the modification of the receptor and the modulation of its ability to influence kinase activity. Homologues of the chemotaxis kinase and regulator proteins are found in many other microbial systems and the entire information processing circuit is analogous to signal transduction systems in eukaryotic organisms. This grant focuses on the continued detailed study of the components of the chemotaxis system and how the properties of the information processing system are derived from the interactions between their components. We will probe the nature of the transmembrane regions of the receptor using mutagenesis and crosslinking to understand the process of transmembrane transmission of information. Functional domains of the transmembrane receptor will be overproduced and their interactions with the kinase coupling protein and other components of the chemotaxis system will be studied in a reconstitution system. We will also reconstitute the effector portion of the pathway and study the interaction of the CheY protein with the flagellar "switch". Details of these processes will be directly applicable to understanding other homologous systems in bacteria and eventually will allow us to understand general "design" strategies involved in biological information processing systems.